Improved efficiency and productivity are constant objectives in modern manufacturing operations. Increasingly, high speed manufacturing has been utilized to help accomplish such objectives. However, high speed manufacturing can be difficult to achieve with conventional devices and can introduce its own attendant problems.
In the area of machine tool operations, high speed manufacturing poses a variety of problems. Conventional machine tool operations utilize ball screw or other mechanical means to drive a workpiece relative to a cutting tool, such as a milling head. These mechanical drives, however, suffer from attendant wear characteristics caused by extended use over prolonged periods of time. Wear associated with mechanical drives becomes increasingly problematic in high speed operations. High speed operations can cause increased and rapid wear to conventional mechanical drives. Under such conditions, mechanical drives require more frequent maintenance, resulting in decreased efficiency and productivity.
To eliminate this attendant wear problem, conventional mechanical drives can be replaced with linear motors. Linear motors are also well suited to high speed operations. However, a drawback of linear motors for machine tool applications has been a relative lack of stiffness to resist perturbations caused by the external forces associated with machine tool operations.
During an ordinary machine tool operation, a workpiece is secured to a platform that is driven mechanically relative to a cutting tool. Various external forces may be exerted on the machine tool platform. The most common is the force associated with the contact between the cutting tool and the workpiece. Conventional mechanical drives have an inherent stiffness or resistance to such external forces, arising from friction between the component parts of the mechanical drive. In contrast, linear motors are driven by electromagnetic field variations. As a result, linear motors lack any inherent stiffness to resist such external forces.
Linear motors can, however, be provided with apparent stiffness. This is accomplished through the use of the motor servo controls to correct for deviations from a predetermined position. Nevertheless, conventional linear motor applications have been able to generate measured stiffnesses only on the order of 10,000-50,000 lbs/inch. Unfortunately, such stiffnesses are well below those necessary to meet the quality control requirements of high speed machine tool applications.
U.S. Pat. No. 4,808,901 issued to Sakamoto discloses a typical control apparatus for a linear motor for use in conjunction with an optical disk drive unit. The control apparatus of the Sakamoto patent utilizes position detecting means to generate a present position signal for the moving coil of the linear motor. Through differentiation of the present position signal, present speed and acceleration signals are generated. Deviations of the moving coil's present position, speed and acceleration from designated target position, speed and acceleration are then calculated and used to generate position, speed and acceleration deviation signals. A control signal is generated based upon these present and deviation signals to properly locate the moving coil of the linear motor. The apparatus also utilizes an inclination angle detector to generate an inclination angle signal due to tilting of the optical disk drive, thereby controlling position independent of the Earth's gravitational field.
Significantly, however, the control apparatus of the Sakamoto patent determines acceleration of the linear motor by differentiation of the motor position signal. Differentiation of inherent error in the present position signal merely compounds such error resulting in inaccurate acceleration determinations. Moreover, the control apparatus of the Sakamoto patent does not specifically address the stiffness problems associated with linear motors as previously discussed. As a result, such a control apparatus is incapable of providing the apparent stiffness to a linear motor to resist perturbations caused by external forces present in ordinary high speed machine tool applications.
U.S. Pat. No. 4,921,365 issued to Sanders et al discloses a high speed shuttle printer. The device utilizes a position detector for determining the position of a linear motor used to drive the shuttle of a dot matrix printer. The position detector may be an accelerometer which measures shuttle acceleration. In such a case, position is determined by integration of an acceleration signal. As with differentiation, integration of inherent error in the acceleration signal merely compounds such error resulting in inaccurate position determinations, often referred to as "drift". Once again, however, the device of the Sanders patent does not specifically address the stiffness problems associated with linear motors as previously discussed. As a result, the device cannot provide the apparent linear motor stiffness necessary to resist perturbations caused by external forces present in ordinary high speed machine tool applications.
U.S. Pat. No. 4,967,293 issued to Aruga et al discloses a multi-positioner magnetic disk storage apparatus having means for reducing mechanical vibration interference between positioners. The device disclosed by the Aruga patent utilizes a vibration sensor, such as an accelerometer, to detect vibrations caused by magnetic disk operation. The device then generates additional vibrations designed to cancel or dampen the vibrations caused by magnetic disk operation. However, the device of the Aruga patent is not designed for force rejection and does not specifically address the stiffness problems associated with linear motors as previously discussed. Once again, therefore, the device cannot provide the apparent linear motor stiffness necessary to resist perturbations caused by external forces present in ordinary high speed machine tool applications.